Adapter to connect Duex4 to the DuetWifi ?

Somewhere was mentioned that the Duex4 is not compatible to the DuetWifi.
Is this just due to different pinout and size on the expansion header ?

In this case a dump adapter would be alow reuse of the Duex4, a benefit for people having it already today and also it means you can sell the Duex4 also for the DuetWifi.
A special ribon cable may do it, or an adapter board from 50-pin to 40-pin.

Yes, it would be possible and basically just a matter of allowing for the different pin assignments. However, the Duet WiFi build of RepRapFirmware does not include support for the digipots used on the DuetX4 to set the motor currents, although you could add that back in. Also missing in the firmware at present is a way of deciding whether drivers beyond the first 5 are TMC2660s, or dumb step/dir/enable drivers - but I will be adding that at some point.

It sounds great that a reuse of the Duex4 is theoretical possible and planned.

As the DuetWifi has on the expansion header signals for an 5 'th stepper, would it than be possible to use the 5'th expansion channel for a complete different stepper driver ?
I'm looking for some powerful driver of 5A which requires Enable, Direction and Step.
Current settings are made with jumper on the driver board, so no support from firmware is required there.

The pin outs on the header are different enough that it would either be a really complicated cable, individual pin-pin cables (no recommended for long term use) or a PCB to provide the pin-pin routing.